I've been starting to dabble with playing with different aspect ratios of my final out put for video. I'm wondering, when you're using a 5D3, what are your best methods to use to make sure you frame the composition in a manner that makes sure you have all your content in frame?

It sure would be cool, if there was a button on the camera that would put the black bars where the 'crop' would be given various aspect ratios that might be used.

What do ya'll do when wanting to make sure your composition is framed properly when you want to use something other than 16:9 ?

Not to oversimplify the situation, and definitely not the most scientific. You could put masking tape (or your adhesive strip of choice) over the rear display and keep everything in between the top and bottom sections.

The best solution (and not necessarily the easiest) is to buy a good anamophic lens, that way there is no loss of resolution from cropping, but it involves correct alignment of the lens and post processing to correct for the aspect ratio.

Is there a measurement you'd use on the screen on the back to know where to put the tape?

If you wanted it fairly accurate,I'd suggest something like framing your cameras screen with a TV playing something 2.35:1,or convert a clip of something to MOV and try playing it on your cameras screen.

I'd cut thin strips of white electrical tape, it should peel off easily and not too sticky.